ICAA CPL Theory key terms

The speed shown on the airspeed indicator, derived from the pressure difference between pitot and static sources. It is not corrected for position error, instrument error, altitude or temperature.

Indicated airspeed corrected for position (installation) error and instrument error. It is the speed used for many aircraft performance charts and approximates true airspeed at sea level in standard conditions.

The actual speed of the aircraft through the surrounding air mass. It equals calibrated airspeed corrected for air density, so it increases with altitude for a given IAS.

The speed of the aircraft relative to the ground, found by combining true airspeed with the wind component. It determines time en route and fuel used over the ground.

The direction in which the aircraft's nose points, measured in degrees from magnetic north. It differs from the intended track when wind causes drift.

The actual path the aircraft follows over the ground, expressed as a direction from a reference north. Wind causes track to differ from heading.

The angular difference between the aircraft's heading and the track it actually makes good over the ground. It results from the crosswind component and is applied to correct heading.

A vector diagram relating true airspeed, wind velocity and groundspeed. It is used to calculate heading, groundspeed and drift for a planned leg.

The angular difference between true north and magnetic north at a given location, caused by the Earth's magnetic field. It is shown on charts and applied when converting between true and magnetic directions.

The error in a magnetic compass caused by magnetic fields within the aircraft itself. It varies with heading and is recorded on a compass correction card.

The angle between the wing chord line and the relative airflow. Increasing it raises lift up to the critical angle, beyond which the wing stalls.

The angle of attack at which airflow separates from the wing and lift begins to decrease sharply. A wing stalls at this angle regardless of airspeed, attitude or weight.

A condition in which the wing exceeds its critical angle of attack, causing airflow separation and a loss of lift. It can occur at any speed or attitude if the critical angle is exceeded.

The minimum airspeed at which an aircraft can maintain level flight in a given configuration and weight. It increases with weight, load factor and aft centre of gravity, and decreases with flaps.

The ratio of total lift to the aircraft's weight, expressed in g. It increases in turns and manoeuvres, raising stall speed and structural stress.

The point at which the aircraft's weight is considered to act. Its position affects stability, control and performance, and must remain within the certified forward and aft limits.

The tendency of an aircraft to return to its original flight condition after a disturbance. It depends on the relationship between the centre of gravity and the aerodynamic centre.

Stability about the aircraft's lateral (pitch) axis. It is influenced by the centre of gravity position and the effectiveness of the horizontal stabiliser.

Drag caused by the generation of lift, arising from the rearward tilt of the lift vector and wingtip vortices. It is greatest at low speed and high angle of attack.

Drag caused by friction and form resistance as the aircraft moves through the air, excluding induced drag. It increases with the square of airspeed.

The aerodynamic force produced perpendicular to the relative airflow by a wing. It depends on air density, airspeed squared, wing area and lift coefficient.

The maximum altitude at which an aircraft can maintain a specified climb rate, usually 100 feet per minute. It is limited by decreasing air density and engine performance.

Pressure altitude corrected for non-standard temperature. High density altitude reduces engine power, propeller thrust and lift, lengthening takeoff and landing distances.

The altitude indicated when the altimeter subscale is set to standard pressure, 1013.25 hPa. It is used for flight levels and performance calculations.

The altimeter subscale setting that makes the altimeter read elevation above mean sea level when on the ground. It is used below the transition altitude and provided by air traffic services.

The altimeter subscale setting that makes the altimeter read height above a specified reference datum, usually the aerodrome. It is used in the circuit and approach in some states.

The altitude below which the altimeter is set to QNH and above which flight levels using standard pressure are used. Its value is published for each aerodrome or area.

The lowest flight level available for use above the transition altitude. It is determined so that adequate vertical separation exists between aircraft using QNH and those using standard pressure.

Rules under which a pilot operates primarily by visual reference to the ground, subject to specified visibility and cloud clearance minima. VFR flights require the aircraft to remain clear of cloud.

Rules governing flight under which navigation and separation are conducted primarily by reference to instruments, with specified equipment and pilot qualification requirements.

Visual meteorological conditions: weather conditions meeting the specified visibility and distance-from-cloud minima that permit flight under visual flight rules.

Instrument meteorological conditions: weather conditions below the specified visual minima, requiring flight under instrument flight rules and instrument reference.

A pilot's incorrect perception of position, attitude or motion relative to the Earth, usually caused by conflicting vestibular and visual cues in poor visual conditions. It can lead to loss of control.

A condition in which insufficient oxygen reaches the body's tissues. In aviation it commonly results from high altitude, and it impairs judgement and vision before the pilot notices symptoms.

The period after loss of oxygen supply during which a person can still perform useful tasks before becoming incapacitated. It shortens rapidly as altitude increases.

Abnormally rapid or deep breathing that reduces carbon dioxide in the blood, causing dizziness, tingling and confusion. Its symptoms resemble hypoxia and it is managed by controlled breathing.

A false sensory perception, often visual, that can mislead a pilot about altitude, distance or runway slope during approach and landing. Awareness and instrument cross-check reduce its effect.

The boundary where a colder air mass advances and undercuts warmer air, producing rapid lifting, cumuliform cloud, showers and often gusty surface winds. It typically passes quickly with a marked wind shift.

The boundary where a warm air mass rides over a retreating colder air mass, producing extensive stratiform cloud and steady precipitation over a wide area. Visibility is often poor.

A narrow band of intense convective activity, often ahead of a cold front, producing severe turbulence, heavy rain, hail and strong gusty winds. It should be avoided by aircraft.

A cumulonimbus cloud system with strong updraughts and downdraughts, lightning, turbulence, icing and hail. Its three stages are cumulus, mature and dissipating.

The accumulation of ice on aircraft surfaces when flying through supercooled water droplets. It increases weight and drag and reduces lift and control effectiveness.

An instrument that measures altitude by sensing static pressure. It displays height above a selected reference datum, depending on the subscale setting used.

The system of pressure sources and lines that supplies the airspeed indicator, altimeter and vertical speed indicator. Blockages cause erroneous indications in these instruments.

A simple instrument that indicates magnetic heading using a floating magnet. It is subject to deviation and turning and acceleration errors in manoeuvres.

A system of lettered airspace classes (A to G) defining the rules, separation and clearance requirements for flights within each volume of airspace.

Airspace within which air traffic control services are provided and clearances are required for certain flights. It includes classes A to E and is depicted on aeronautical charts.

An authorisation from air traffic control for an aircraft to proceed under specified conditions. It does not relieve the pilot of responsibility for safety or terrain clearance.

The pilot's repetition of safety-related elements of an ATC clearance to confirm correct reception and understanding. It is required for clearances, runway assignments and frequency changes.

The ICAO spelling alphabet used to transmit letters unambiguously over radio, such as Alpha, Bravo and Charlie. It reduces errors in communication caused by poor reception.

The international distress call indicating imminent danger requiring immediate assistance. It is spoken three times at the start of a distress transmission.

The international urgency call indicating a situation concerning safety that does not require immediate assistance. It is spoken three times and has priority over routine traffic.

A condition of being threatened by serious or imminent danger requiring immediate assistance. It is communicated with the distress call Mayday.

A four-digit code set on the aircraft transponder to identify the flight to air traffic control radar. Specific codes are assigned for emergencies and radio failure.